It is well known that most of the bodily states in mammals including most disease states, are effected by proteins. Such proteins, either acting directly or through their enzymatic functions, contribute in major proportion to many diseases in animals and man. Classical therapeutics has generally focused upon interactions with such proteins in efforts to moderate their disease causing or disease potentiating functions. Recently, however, attempts have been made to moderate the actual production of such proteins by interactions with molecules that direct their synthesis, intracellular RNA. These interactions involved the binding of complementary "antisense" oligonucleotides or their analogs to the transcellular RNA in a sequence specific fashion such as by Watson-Crick base pairing interactions.
The pharmacological activity of antisense oligonucleotides, as well as other therapeutics, depends on a number of factors that influence the effective concentration of these agents at specific intracellular targets. One important factor is the stability of the oligonucleotide in the presence of nucleases. Another key factor is the ability of antisense compounds to traverse the plasma membrane of specific cells involved in the disease process.
Cellular membranes consist of lipid protein bilayers that are freely permeable to small, nonionic, lipophilic compounds and inherently impermeable to most natural metabolites and therapeutic agents. Wilson, D. B. Ann. Rev. Biochem. 47:933-965 (1978). The biological antiviral effects of natural and modified oligonucleotides in cultured mammalian cells have been well documented, so it appears that these agents can penetrate membranes to reach their intracellular targets. Uptake of antisense compounds into a variety of mammalian cells, including HL-60, Syrian Hamster fibroblast, U937, L929, CV-1, and ATH8 cells has been studied using natural oligonucleotides and nuclease resistant analogs, such as alkyl triesters, Miller, P. S., Braiterman, L. T. and Ts'O, P. O. P., Biochemistry 16:1988-1996 (1977); methylphosphonates, Marcus-Sekura, C. H., Woerner, A. M., Shinozuka, K. Zon, G., and Quinman, G. V., Nuc. Acids Res. 15:5749-5763 (1987) and Miller, P. S., McParland, K. B., Hayerman, K. and Ts'O, P. O. P., Biochemistry 20:1874-1880 (1981); and phosphorothioates, Ceruzzi, M. and Draper, K. Nucleosides & Nucleotides 8:815-818 (1989 ); Miller, P. S., Braiterman, L. T. and Ts'O, P. O. P. Biochemistry 16:1988-1996 (1977) and Loke, S. L., Stein, C., Zhang, X. H. Avigan, M., Cohen, J. and Neckers, L. M. Curr. Top. Microbiol. Immunol. 141:282-289 (1988).
Phophorothioates are oligonucleotide analogs in which the oxygen atom in each phosphate linkage is replaced by a sulfur. Although the overall charge is conserved, and they are therefore comparable in that respect to phosphodiester oligonucleotides, several properties of this class of analogs makes them more attractive than other modified compounds. These include ease of chemical synthesis, good aqueous solubility, relatively high resistance to nucleases, and the ability to form stable duplexes with complementary DNA or RNA strands. However, phosphorothioates were studied concurrently with natural compounds by Loke et al, Proc. Natl. Acad. Sci. U.S.A. 86:3473-3478 (1989), and while they may be useful due to their nuclease resistance, they are less efficiently internalized than their natural oligonucleotide counterparts.
Advances in nucleotide chemistry have allowed attachment of functional groups to the 3' and 5' end of the oligonucleotides to enhance cellular uptake in specific cell types. Previous studies have shown that plasmid DNA complexed with an asiaglycoprotein-poly(L-lysine) conjugate, could be targeted to hepatocytes, which contain unique cell surface receptors for galactose-terminal (asialo)glycoproteins. Wu, G. Y. and Wu, C. H. Biochemistry 27:887-892 (1988). Other groups have synthesized oligodeoxyribonucleotides that have a 5'-attached alkylating agent and a 3' attached cholesterol moiety and determined that these modified oligonucleotides were taken up into cells more efficiently than control compounds without the steroid moiety. Zon, G. in Oligodeoxynucleotides: Antisense Inhibitors of Gene Expression 234-247, ed. J. S. Cohen (CRC Press, Boca Raton FL, 1989). Letsinger, et al. Proc. Natl. Acad. Sci. U.S.A. 86:653-656 (1989), have also synthesized cholesteryl-conjugated phosphorothioates whose anti-HIV activity is significantly greater than natural oligonucleotides with the same sequence. Additional modifications include conjugation of oligonucleotides to poly(L-lysine) alone. Stevenson, M. and Iversen, P. L. J. Gen. Virol 70:2673-2682 (1989 ) and Lemaitre, M., Baynard, B. and LeBleu, B. Proc. Natl. Acad. Sci. U.S.A. 84:648-652 (1987). This modification enhanced the antiviral activity of the compound studied presumably due to increased cellular uptake imparted by the polycationic poly(L-lysine).
The activity of antisense oligonucleotides previously available has not been sufficient for practical therapeutic, research or diagnostic use. The basis of this insufficiency is likely several fold i.e., (1) incomplete understanding of the secondary and tertiary structure of the targeted RNA, (2) low percentages of delivery and uptake, (3) inactivation of reactive centers by other cellular components, and (4) requirements for stoichiometric conditions for inhibition of protein production.
Enhancement of cellular uptake of antisense oligonucleotides by chemical modification would have clear advantages. Novel modifications may also lead to increased lipophilicity, greater retention, and increased distribution of the novel compounds. Increasing the concentration of oligonucleotides at specific intracellular target sites may ultimately increase the safety and efficacy of these compounds since less of the drug will be required to produce the desired effects.
Accordingly, there has been and continues to be a long-felt need for oligonucleotides and oligonucleotide analogs which are capable of effective therapeutic and diagnostic antisense use and specifically an oligonucleotide or oligonucleotide analog which is comprised of a functional group which facilitates transport into the cell and at the same time is less susceptible to nuclease activity than wild types. This longfelt need has not been satisfied by prior work in the field of antisense oligonucleotide therapy and diagnostics.